This invention relates to charged particle beam recording on film and more specifically to a system for accurately tracing with an electron beam on an electron sensitive film. Such a system finds use in recording graphic images where a high degree of resolution and accuracy of placement is needed. Thus, electron beam recording is highly useful for, among other things, television recording, computer output microfilm, mass data storage, wide bandwidth signal recording, satellite photography, automated cartography, computer micrographics and seismic recording.
Essential elements of an electron beam recorder (EBR) are an electron gun, an electro-optical focusing and deflection system, an electron sensitive film and a film gate. In operation, the electron beam traces an image across the recording area on the emulsion side of a stationary film in the film gate. Because scattering and attenuation of the electron beam occurs in air, electron beam recording must be conducted in a high vacuum. Generally, a three stage vacuum system is used in electron beam recorders with the highest vacuum in the vicinity of the electron gun, a lower vacuum in the vicinity of the emulsion side of the film and the lowest vacuum in the vicinity of the back of the film. Such a three stage system allows relatively rapid access to the film for changing and replacement thereof. However, because of the differential pressure exerted on the film by the three stage system, buckling of the film occurs. This buckling is highly variable and unpredictable. Moreover, by placing the film in a vacuum, outgassing of water vapor and other atmospheric constituents is induced. This outgassing occurs at differential rates in the emulsion and film base and also results in a highly variable and unpredictable buckling.
It is recognized that positional uncertainties of the recording film are the primary limitations to the geometric accuracy of images recorded in high performance EBRs.
Conventional EBR film gates typically comprise an extremely flat platen surface which clamps the periphery of the film against a flat framing member. Due to the aforesaid buckling factors, although the film is fixed about the perimeter of the frame, it does not maintain good contact with the flat platen surface and an unknown and variable buckle is present in the film. For example, in the typical operation of a conventional large format EBR having a recording aperture of approximately 8.times.5 inches, the recording film may become spaced from the flat surface by anywhere from 0.010 to 0.030 inches in the central portion of the recording area. In other film recording systems operating at normal atmospheric pressure film buckle can be removed by the application of suction to the back of the film through small apertures in the flat surface of the film gate. However, such an approach is inapplicable in a vacuum system which is necessary for EBR operation.
The electron sensitive film used in EBR's conventionally includes a charge dissipating layer since film not provided with special charge dissipating means acquires a static electric charge when exposed to an electron beam. The electric field resulting from this static charge causes spurious beam displacements. By constructing the film gate from an electrically conducting material and maintaining the film in intimate contact with the film gate in accordance with the present invention, the electric field due to the static charge acquired by the film can be lessened without provision of auxiliary charge dissipating means. Because of the film buckling problem of prior film gates, they cannot achieve the intimate contact necessary to attain lessening of the electric field.